workload/schemachanger: fix ALTER COLUMN SET NOT NULL flakes

Previously, ALTER COLUMN SET NOT NULL could flake if a concurrent INSERT
happened during this operation, when executing under the legacy schema
changer. To address this, this path allows null violations to be a
potential execution error, if the operation is run in the legacy schema
changer.

Fixes: #135692

Release note: None

pkg/workload/schemachange/operation_generator.go

@@ -2465,8 +2465,12 @@ func (og *operationGenerator) setColumnNotNull(ctx context.Context, tx pgx.Tx) (
 		}
 		if colContainsNull {
 			og.candidateExpectedCommitErrors.add(pgcode.NotNullViolation)
-			// If executed within the same txn as CREATE TABLE.
-			stmt.potentialExecErrors.add(pgcode.NotNullViolation)
+		}
+		// If we are running with the legacy schema changer, the not null constraint
+		// is enforced during the job phase. So it's still possible to INSERT not null
+		// data before then.
+		if !og.useDeclarativeSchemaChanger {
+			og.potentialCommitErrors.add(pgcode.NotNullViolation)
 		}
 	}